The study included 11,178 7-year-olds living in the U.K. who were tracked from infancy. Their parents were asked about bedtimes when the children were ages 3, 5, and 7.
The effect was most striking in three year olds, where boys and girls scored lower on reading, math and spatial skills tests than children of the same age who kept to a regular schedule.
An irregular bedtime at age 5 was associated only with lower scores for reading among girls and for math among boys.
The differences in test scores were modest -- often only a few points -- but irregular bedtimes throughout childhood appeared to have a cumulative effect. By age seven, both boys and girls without regular bedtimes performed lower.
Scientists at University College London said the lack of routine might disrupt the body clock leading to impaired development. Late bedtimes in general contributed to sleep deprivation, which affects the brain's ability to process information and form memories.
Writing in the Journal of Epidemiology and Community Health, the authors suggest that irregular bedtimes affect the brain's "plasticity," or ability to learn and retain new information.
"Sleep is the price we pay for plasticity on the prior day and the investment needed to allow fresh learning the next day," the authors wrote. "Therefore, reduced or disrupted sleep, especially if it occurs at key times in development, could have important impacts on health throughout life."
"It might be that inconsistent bedtimes are a reflection of chaotic family settings and it is this, rather than disrupted sleep that impacts on cognitive performance in children," the authors acknowledged. "However, we found that inconsistent bedtimes were linked to markers of cognitive performance independent of multiple markers of stressful family environments."
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